Motor vehicles currently use semiconductor devices to perform a wide variety of functions. Engine control circuits use microprocessors to monitor the engine's state and control its operation. Information collected from various sensors in the engine is collectively analyzed and processed to produce timing and control signals for operating engine subsystems. One such subsystem includes an ignition circuit that receives timing signals to control a power switching transistor that supplies current to charge an ignition coil. When the power transistor is turned off, the coil voltage rises to a level sufficient to fire a spark plug, which discharges the coil.
A problem with ignition circuits occurs if the spark plug is removed or fouled so that no spark is generated. Since there is no spark to discharge the coil, the coil voltage can rise to an excessive level, creating an overvoltage condition that causes the coil energy to be dissipated in the power transistor. The dissipation can stress or damage the power transistor and degrade the reliability of the ignition circuit. Previous ignition circuits include internal limiting circuits to limit the voltage and power dissipation while shutting down the ignition circuit. However, the limiting circuits are activated automatically with timing networks, and therefore operate only at specific times in an ignition cycle. If a fault such as an excessive coil temperature occurs at a different time, damage could still result.
Hence, there is a need for an ignition circuit which provides a shut down method that can be activated whenever a fault condition is detected in order to reduce damage and improve the reliability of the ignition circuit.